In recent times bit rate reduction techniques have been employed to increase transmission capacity over digital transmission facilities. One such technique is adaptive differential pulse code modulation (ADPCM). ADPCM is employed to increase capacity over voiceband digital transmission facilities. Use of 32 kilobit/sec ADPCM is increasing and, normally, doubles the capacity of T carrier facilities. Greater transmission capacity may be realized by judiciously transmitting the voiceband signals at still lower bit rates than the 32 kilobit/sec rate.
The 32 kilobit/sec rate ADPCM, however, presents a problem when transmitting certain non-voice signals. Typically, non-voice signals, for example, voiceband data signals, are transmitted at the 32 kilobit/sec rate ADPCM. That is, no bits are allowed to be dropped to lower the transmission bit rate. When transmitting "higher" bit rate voiceband data signals, for example, those generated by a 9600 bit/sec or higher rate modem, the use of the 32 kilobit/sec so-called fixed rate ADPCM results in unacceptable bit error rates. Consequently, the data must be retransmitted thereby resulting in unacceptable transmission throughput. In order to minimize this problem it is desirable to transmit the 9600 bit/sec and higher rate voiceband data signals at an ADPCM transmission bit rate or other PCM transmission bit rates higher than the present fixed ADPCM bit rate of 32 kilobit/sec. Additionally, it may be acceptable and desirable to transmit voiceband data signals having "lower" bit rates at a bit rate less than the 32 kilobit ADPCM. In order to effect transmission of the voiceband data signals at bit rates higher or lower than the 32 kilobit/sec ADPCM rate, they must be classified as to their respective baud rates and/or modulation scheme.
Heretofore, attempts at classifying voiceband data signals have used a so-called ordinary autocorrelation of the signal. A problem with the use of the ordinary autocorrelation is that the results are modulated by the carrier frequency of the data signal. Consequently, the results of such a classifying arrangement do not accurately reflect the baud rates or type of modulation of the voiceband data signals.